1. Field of the Invention
This invention relates to a method for estimating the quality of an information reproduction apparatus or the quality of an information recording medium. It also relates to an information recording medium that satisfies a predetermined estimation standard. It further relates to an information reproduction apparatus for reproducing information stored in an information recording medium that satisfies a predetermined estimation standard.
2. Discussion of the Background
In recent years, DVD systems have been commercially produced to meet the demand for recording MPEG2 images on one surface of an optical disk of 12 cm diameter for two hours or more. According to the DVD standards, the memory capacity, the track density and the linear density of one surface of a disk are 4.7 GB, 0.74 μm/track and 0.267 μm/bit, respectively. Any DVD based on the DVD standards will hereinafter be referred to as a “currently available DVD”.
An optical head incorporated in a DVD reproduction apparatus reproduces information stored in on optical disk such as a DVD. The optical head includes an LD (Laser Diode), an object lens, a condensing lens and a photodetector, etc. A light beam emitted from the LD converges, through the object lens, onto a series of pits formed in a track on the optical disk. A light beam reflected from the optical disk converges onto the photodetector through the condensing lens, and is converted into a reproduction signal. The reproduction signal from the photodetector is input to a reproduction signal processing system, is then subjected to waveform equalization in an equalizer, and is decoded into data in a detector. In the case of the DVD standards, the LD of the optical head has a wavelength of 0.65 μm, and the object lens has a numerical aperture of 0.6.
The currently available DVD reproduction apparatus generally uses the waveform slice method as a reproduction signal processing method. Suppose that the optical head incorporated in the currently available DVD reproduction apparatus reproduces information stored not in a currently available DVD, but in a next generation DVD which has a higher density recording capacity. Since the next generation DVD has a higher track density, a reproduction signal output from this DVD contains a large signal-degrading crosstalk component. Further, the higher the linear density of the DVD, the flatter the waveform of the reproduction signal. The equalizer, which amplifies the high frequency component of the reproduction signal, needs to amplify the high frequency component to a high degree when the waveform of the input reproduction signal is not so sharp. This process results in the amplification of the signal-degrading component, too. In the case of using the waveform slice method as the signal detection method, the amplification of the signal-degrading component is inevitable when the recording density of the apparatus is high, and hence correct data decoding cannot be executed.
In place of the waveform slice method, the partial response and maximum likelihood (PRML) method is proposed as a reproduction signal processing method employed when the reproduction signal has a low signal to noise ratio (SNR). In the PRML method, at first, the equalizer equalizes the waveform of a reproduction signal into a waveform having a predetermined correlation therewith, between recognition positions called “partial response characteristic” (PR characteristic) positions. The reproduction signal processing system using the PRML method generally employs a Viterbi decoder, a typical maximum likelihood decoder, as a detector located after the equalizer. If the equalizer equalizes the reproduction signal waveform into a waveform of, for example, PR (1, 2, 2, 1) characteristic (explained below), the Viterbi decoder selects the series of signal values of the equalization signal that minimizes the difference between a series of sample values of the equalization signal and a series of ideal values of the equalization signal, and outputs binary data (decoded data) corresponding to the selected series.
Sample values of an equalization signal are not necessarily identical to ideal sample values because of, for example, noise. The difference between a sample value and an ideal sample value is called “equalization error”. In the PRML method, this difference is used as an estimation reference value for estimating the quality of an optical disk medium or adjusting an optical disk apparatus. To obtain the equation error, the ideal sample value is necessary. Japanese Kokai Publications Nos. 7-235150 and 8-195037, which are incorporated herein by reference in their entirety, disclose techniques for calculating an ideal sample value on the basis of the decoded data obtained by Viterbi decoding.
If the information recording/reproduction system that uses the PRML method for processing a reproduction signal employs the above-described conventional method using a recognition result in order to calculate an estimation reference value for estimating the quality of a recording medium or adjusting the circuit in the apparatus, it must incorporate a recognition circuit and a timing adjustor circuit. In other words, the system inevitably has a large size and a complicated structure.